A known image reader for reading a document is mounted on a facsimile, a copier, a scanner, or any combination thereof. In the known image reader, an image sensor includes light receiving elements which are linearly arranged. The image sensor receives a reflection light from the document, and performs photoelectric conversion. A close-contact-type image sensor (“CIS”) is one type of image sensor which is popular because it is relatively lightweight and may be manufactured at a reduced cost. Nevertheless, because a focal length of the close-contact-type image sensor is relatively short, when a lens mounting portion exhibits poor flatness, the resolution is deteriorated. Moreover, when a distance from a lens surface to a platen is individually adjusted for each image reader, the number of manufacturing steps and the manufacturing cost increases.
It is known that an image reader may calculate a Modulation Transfer Function (“MTF”), i.e., the change of contrast of a sinusoidal pattern image expressed as a function of spatial frequency, based on a maximum value and a minimum value of image data read from a stripe band, and may determine an edge emphasis gain and a smoothing coefficient based on a result of the calculation. Consequently, the MTF of an image may be enhanced without increasing the accuracy of mechanical members and without individually adjusting the distance from the leis surface to the platen for each image reader.
Nevertheless, in the known image reader, the edge emphasis is performed or the smoothing coefficient is determined uniformly with respect to the entire close-contact-type image sensor. Therefore, when accurate focusing is not obtained partially, the accurate focusing may not be performed at such a portion, or the accurate focusing may not be performed at any portions.
Moreover, the close-contact-type image sensor of an A3 size or the like uses a thick and rigid frame for preventing the frame from warping. Specifically, when the image sensor comprises a frame comprising plastic, which is light weight and has a low rigidity, the accurate focusing is not obtained partially because of warping. Accordingly, the image sensor comprises thick and rigid frame. Nevertheless, such a frame is not suitable for miniaturized, light weight close-contact-type image sensor.